U.S. university researchers say the application of a specific species of fungi into the concrete matrix during the mixing process shows promise in healing cracks in concrete before they lead to corrosion of the reinforcing steel rebar.
“Without proper treatment, cracks tend to progress further and eventually require costly repair,” says Congrui Jin, an assistant professor at Binghamton University (Binghamton, New York, USA) who has worked on the problem since 2013 and recently issued a paper with findings.1
“If micro-cracks expand and reach the steel reinforcement, not only the concrete will be attacked, but also the reinforcement will be corroded, as it is exposed to water, oxygen, possibly CO2 [carbon dioxide], and chlorides, leading to structural failure.”
In research thus far, Jin’s team says it has found that calcium carbonate [CaCO3] microbiologically induced through the spores of pH regulatory mutant fungi can offer both a cost-effective and environmentally friendly means to remove inorganic contaminants from concrete in situ.
Origins of the Concept
In their research, Jin worked with Binghamton professor colleagues Guangwen Zhou and David Davies, along with Rutgers University (New Brunswick, New Jersey, USA) associate professor Ning Zhang to explore a self-healing concept in which fungi are used as a self-healing agent to promote calcium mineral precipitation to fill the cracks in concrete.
The professors also received support and funding from the Sustainable Communities research foundation within the Transdisciplinary Area of Excellence program at the State University of New York (SUNY) (Albany, New York, USA).
“This idea was originally inspired by the miraculous ability of the human body to heal itself of cuts, bruises, and broken bones,” Jin explains. “For the damaged skin and tissue, the host will take in nutrients that can produce new substitutes to heal the damaged parts.”
Within concrete’s porous structure, steel corrosion often occurs from either the aforementioned ingress of chloride ions (Cl-) or carbonation,2 where CO2 from the atmosphere penetrates the concrete through cracks over a prolonged period of time and changes the concrete’s alkalinity. In many circumstances, corrosion of the reinforcing steel is caused by an eventual failure of a protective passive oxide layer from chloride ions, or the steel’s inability to form a passive layer because the surrounding concrete is no longer alkaline. If the concrete cover that protects the steel is damaged via a crack and the bond between the concrete and steel reinforcing bar is broken, the steel’s passive layer can break down and active corrosion of the steel begins.
The passive oxide layer on the reinforcing steel is formed by hydroxyl ions (OH-) in the pore water. These OH- ions form due to high alkalinity, with a pH level of 11 or higher. When Cl- reach the steel, they can compete with the OH- that form the protective layer. In areas on the steel surface with low amounts of OH-, Cl- can also cause pitting. Thus, a structure with additional OH- (due to initial water or oxygen penetration) stands a better chance to resist the Cl- ions—as long as any subsequent chloride ingress is held in check.
According to the researchers, fungi have long shown promise in forming calcium precipitates, which can seal cracks from within and thus protect against chloride-induced corrosion and carbonation over time. While concrete is inherently porous, sealing cracks delays the eventual ingress of chlorides. The problem has been that prior studies showed that most fungi could not survive under the highly alkaline and lofty pH levels often found in concrete, the researchers explain. In other words, the water and oxygen ingress would eventually be followed by chlorides, since the fungi were usually unsuccessful in forming calcium. In turn, the cracks would remain open.
Thus, the researchers began studying certain species of fungi to figure out which, if any, could grow when exposed to water and oxygen from cracks while also surviving the resulting high pH. This would give them the best of both worlds by simultaneously boosting the protective passive oxide layer while also filling the cracks to further slow the penetration of longer-term corrosive agents, such as chlorides.
“The fungal spores, together with nutrients, will be placed into the concrete matrix during the mixing process,” Jin explains. “When cracking occurs, water and oxygen will find their way in. With enough water and oxygen, the dormant fungal spores will germinate, grow, and precipitate calcium carbonate to heal the cracks.”
“When the cracks are completely filled and ultimately no more water or oxygen can enter inside, the fungi will again form spores,” he adds. “As the environmental conditions become favorable in later stages, the spores could be wakened again.”
Trichoderma reesei Species
An initial screening of different species of fungi was conducted, with fungal growths overlaid onto cured concrete plates. Mycelial discs were aseptically deposited at the center of each plate. When using the Trichoderma reesei fungus, researchers discovered that the fungus in the concrete remained in a dormant state until the concrete sustained its first crack. At that point, the species displayed characteristics that “healed” cracks in the laboratory setting—even when exposed to sudden and rapid pH growth.
X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques confirmed that the crystals precipitated by the fungus were composed of calcite, according to the researchers.
In the case of this fungus, results showed that due to the leaching of calcium hydroxide [Ca(OH)2] from concrete, the pH of the growth medium increased from its original value of 6.5 to 13.0. Nonetheless, in the lab environment, the cracks were still sealed at those pH levels. So besides sealing the cracks and slowing the ingress of Cl- or carbonation, this method also helps the formation of OH- ions—which provide the passive oxide layer for additional corrosion protection.
Future Research Steps
Going forward, researchers say the biggest challenge for their project involves the survivability of the self-healing fungi in concrete in more severe environments outside of the laboratory. However, they are hopeful that with further adjustments, the fungi will be able to effectively fill cracks.
“There are still significant challenges to bring an efficient self-healing product to the concrete market,” Jin says.
Specifically, the researchers say they plan to study the effects of various factors influencing fungal calcium precipitation, including pH, temperature, ultraviolet light, growth medium composition, and fungal spore concentration. For example, they plan to introduce varying concentrations of calcium and inorganic phosphate to the growth medium to examine the effects on carbonate precipitation.
“In my opinion, further investigation in alternative microorganisms such as fungi and yeasts for the application of self-healing concrete becomes of great potential importance,” Jin says.
Source: Binghamton University, www.binghamton.edu. Contact Congrui Jin, Binghamton University—email: firstname.lastname@example.org.
1 “Using Fungi to Fix Bridges,” Binghamton University News, Jan. 8, 2018, https://www.binghamton.edu/news/story/938/using-fungi-to-fix-bridges (Feb. 12, 2018).
2 K. Riggs Larsen, “Corrosion Effects on the Durability of Reinforced Concrete Structures.” MP, Nov. 30, 2015, http://www.materialsperformance.com/articles/material-selection-design/2015/12/corrosion-effects-on-the-durability-of-reinforced-concrete-structures (Feb. 12, 2018).